The Role of Dissolved Organic Material in the Nutrition of Pelagic Larvae: Amino Acid Uptake by Bivalve Veligers

SYNOPSIS. Studies on the role of dissolved organic materialin the nutrition of marine invertebrates have largely been confinedto adults. However larval forms, with a higher surface areato volume ratio, have a greater weight specific capacity forabsorbing dissolved organic material than adults. Autoradiographic,biochemical, and kinetic experiments with bivalve larvae allindicate that amino acid uptake and translocation mechanismscan operate efficiently at naturally occurring substrate concentrations.The mechanisms operate throughout the life-span of the animal,from fertilized egg to adult. Experimental evidence is presentedto show that the kinetics of uptake by larvae allow them tocompete with bacteria for dissolved organic material in seawater. In larvae, supplementary sources of energy may be moreimportant than in adults since larvae are provided with minimalfood reserves by the parent and must pass through periods whenpaniculate feeding cannot occur.     

Adaptations by Invertebrate Larvae for Nutrient Acquisition from Seawater

Unlike life on land, animals that live in seawater are surroundedby a medium that contains organic nutrients in dilute solution.Larval forms of soft-bodied marine invertebrates are adaptedto take advantage of the fact that most of the organic carbonin their environment is in solution as dissolved organic material(DOM). New evidence for the importance of DOM to metazoans ispresented by showing that larval forms can increase in biomass,even in the absence of paniculate foods. Such increases occurredonly in those species capable of transporting DOM. The physiologicalbasis for using DOM as an energy source is dependent upon anincreased transport capacity for DOM as growth proceeds. Usingbivalve larvae, mass coefficients and exponents were determinedfor (i) alanine transport rates and (ii) metabolic rates. Thesecoefficients were not statistically different when determinedover the life span of a larva. Thus, as growth proceeds, theselarvae increase their ability to obtain a potential supply ofmetabolic fuel (DOM) in direct proportion to the increase intheir metabolic demand. The percent of this increased transportcapacity that larvae could actually utilize in nature will dependupon the substrate concentrations in their environment. Currentviews on what these concentrations are in seawater may be alteredas more attention is given to the fine scale distributions oforganic chemicals in the ocean. After DOM has been transportedby the animal, its metabolic fate can now be rigorously studiedusing bacteria-free larvae. Measurements of amino acid synthesisin larvae cultured under axenic conditions suggest that a muchgreater plasticity may exist in the biochemical requirementsof larvae for dietary amino acids.